The marine CSEM exploration method uses a man-made source to generate electromagnetic (“EM”) waves and deploys receivers on the seafloor to record EM signals. The recorded EM signals are analyzed to infer sub-seafloor structures and/or determine the nature of particular structures, such as reservoirs. FIG. 1 shows the principle of marine CSEM exploration with a Horizontal Electric Dipole (HED) source 12. A tow cable (and control umbilical) 11 pulls the source through the water. Autonomous receivers 13 are located on the sea floor 14 along or near the source tow line (not shown in FIG. 1 but see 33 in FIG. 3). This technology has been applied in tectonic studies, hydrocarbon and mineral exploration, environmental and geological engineering (Chave, et al., in Electromagnetic Methods in Applied Geophysics (ed. M. N. Nabighian), Vol. 2, 931-966, Society of Exploration Geophysicists (1991); Constable and Cox, J. Geophs. Res. 101, 5519-5530 (1996); MacGregor, et al., Geophy. J. Int. 146, 217-236 (2001); Ellingsrud, et al., The Leading Edge, 972-982 (2002); and Eidesmo, et al., First Break 20.3, 144-152 (2002)).
To date, marine CSEM applications involve towing an HED above the seafloor at a slow constant velocity. Typical altitudes (above the seafloor) range between 25 and 75 meters, depending on the length of the dipole and seafloor topography. Typical tow velocities range between 1.0 and 2.0 knots. It is desirable for a marine CSEM source HED to remain:
Horizontal (i.e. the “pitch” angle 21 between the head-tail electrode chord 22 and the horizontal 23, as illustrated in the vertical profile view of FIG. 2, should be small);
Positioned over the pre-plot line (the planned path of the source; it should be noted that the surface vessel will typically not follow the tow line due to surface currents, wind or seafloor bottom currents); and
At the same azimuth as the pre-plot sail line (i.e. the “yaw” angle 31 between the head-tail electrode chord 32 and the pre-plot sail line azimuth 33, as illustrated in the top or plan view of FIG. 3 should be small).
Satisfying these criteria will ensure the maximum transmitted EM energy associated with the horizontal component of the source dipole moment in the desired orientation and zero or negligible transmitted EM energy associated with the across-line 41 and vertical 42 dipole moment components, as illustrated in FIG. 4. (The source dipole moment is a vector equal to dipole length vector multiplied by the transmitted current.) Ideally, the dipole moment would be aligned with the in-line axis (in a horizontal plane, along the pre-plot sail line azimuth). In practice, the source dipole moment 40 will deviate from the preferred direction 44, but the objective is that the in-line component 43 should be as large as possible. The effect of dipole yaw and pitch on transmitted EM signal components are illustrated in FIGS. 17A-B and 18A-B, respectively. FIGS. 17B and 18B illustrate the reduction (percentage of the ideal dipole moment) in the in-line horizontal transmitted EM energy with yaw angle (17B) and pitch angle (18B). FIGS. 17A and 18A show the introduction of transmitted EM energy in an orthogonal component based on a yaw (17A) or pitch (18A) angle. The reduction in the horizontal component may be considered minimal (approximately 13% for a deviation of 30 degrees), the orthogonal component becomes significant (50% for a deviation of 30 degrees). Further, as can be seen from FIGS. 17A and 18A, the deviation in the orthogonal component changes with the direction of the deviation. Any EM signals transmitted in the across-line or vertical components can result in incomplete parametric data processing and interpretation. This follows because the modeling assumes a perfectly aligned dipole. Any vertical response is assumed to be due to the sub-structure. Any vertical dipole moment can contaminate or even mask the predicted response.
Current marine CSEM sources include the following general design features (please refer to FIGS. 2, 3), which are collectively referred to as a “source dipole”:
A “tail drogue” 24, which provides a limited amount of drag to ensure the dipole streams behind the “head fish” 25. The tail drogue may include positioning sensors and is equipped with sufficient flotation to ensure neutral buoyancy.
Head 26 and tail 27 electrodes, which are typically constructed from aluminum or copper. The electrodes are sized (in diameter, length and surface area) to ensure optimal transfer of the EM signal to the surrounding seawater.
A streamer 28, consisting of an electrical conductor(s) which transmits current from the head fish to the electrodes. Generic tethers that may be customized into marine CSEM streamers may be purchased from, for example, South Bay Cable Corporation (www.southbaycable.com). Flotation, either built into the streamer core and/or attached externally, ensures the entire streamer is neutrally buoyant.
A “head fish” 25, which contains the sub sea high power electrical transformer and control electronics (“waveform source”). The head fish is towed by the tow umbilical 11 and in turn tows the “head” and “tail” electrodes, streamer and “tail drogue”. The head fish is very heavy (2,000 lbs or greater) compared to the other elements of the CSEM source.
The marine CSEM source is “flown” to maintain a constant head fish altitude above the seafloor. An operator on the surface survey vessel will “pay-out” or “reel-in” umbilical cable to maintain the desired altitude. The umbilical cable 11 will not follow a straight line chord between the surface vessel's “A-frame” and the head fish due to the effects of the cable drag 53 and the weight 54 of the cable, and the drag 55 and weight 56 of the head fish, as illustrated in FIG. 5. The drag forces are a function of speed. Vector 57, representing the tow force exerted by the vessel, completes the force diagram.
In FIGS. 6 and 7, modeled responses of the umbilical catenary illustrate the changes in head fish position (layback distance and depth) due to changes in the umbilical length (FIG. 6) and surface vessel speed (FIG. 7). Both the depth and layback distance of the head fish will change if the umbilical length or vessel speed change. The primary result of a change in the umbilical length will be a change in depth, with a smaller change in layback distance (FIG. 6). The primary result of a change in tow speed will be a change in layback distance, with a smaller change in depth (FIG. 7).
FIG. 6 illustrates the catenary present in the umbilical. The plotted circles represent an umbilical of length 2,900 m; the triangles represent an umbilical of length 3,000 m; and the squares represent an umbilical of length 3,100 m. The variation in umbilical length (100 m) is greater than the resultant depth change of the head fish (91.9 m). A tow vessel speed of 1.0 knot was used for the simulations of FIG. 6.
FIG. 7 illustrates the change in head fish depth as a function of surface vessel speed for an umbilical length of 3,000 m. The plotted circles represent a tow speed of 0.95 knot, the triangles 1.00 knot, and the squares 1.05 knots. The depth differences are shown on the drawing.
The head fish will respond to changes in surface vessel speed or umbilical length quickly due to the associated vertical forces (upward force through the umbilical or gravity on the very heavy head fish). The streamer is designed to be neutrally buoyant and the main force acting on the streamer is the tow force from the head fish. The tow force is nominally horizontal in direction, with slight deviations from the horizontal when the head fish's altitude changes (due to changes in umbilical length or tow surface vessel speed). The neutrally buoyant streamer will exhibit a damped response to changes in the head fish altitude, which will result in a source dipole pitch angle (FIG. 2).
The surface vessel will “heave” up and down due to both wind and swell generated wave action. The head fish, coupled directly to the vessel's stern through the umbilical, will oscillate with the ship's motion. The amplitude and phase of the head fish oscillations relative to the vessel's motion will depend mostly on the length of umbilical paid out. Other factors affecting response characteristics are vessel oscillation frequency, vessel speed, sea currents, head fish mass, and umbilical physical properties (e.g., mass, diameter, and drag). The umbilical will act like a spring as the vessel oscillates at the sea surface, both due to the umbilical's physical properties and the catenary shape of the umbilical through the water column. This “spring” constant will change as the previously stated factors change. As a general case, the oscillation amplitude of the head fish relative to the vessel decreases as the umbilical cable length increases. Based on the previous discussions, heave induced vertical motion of the head fish will generate variable pitch angles in the marine CSEM source dipole.
The streamer is subject to the following three forces:
1. The tow force through the tow umbilical and head fish,
2. A drag force 82 (see FIG. 8) from sub-sea components being towed through the water,
3. Any seafloor bottom current. FIG. 8 illustrates the effect of a broadside current 81 on the streamer 28 and the resultant yaw angle 31. The magnitude of the yaw angle will be dependent on the seafloor bottom current vector 81 (magnitude and direction).
The presence of a yaw angle will result in an across-line component of the source dipole moment (41 in FIG. 4). A non-zero across-line source component can result in incomplete parametric data processing and interpretation.
Heave Compensation
Several different approaches have been adopted to attenuate or eliminate the affect of surface vessel heave on towed sub-sea vehicles, including:
I. Heave Motion Compensation Winch (141 in FIG. 13) Systems
Adamson (see “Efficient Heave Motion Compensation for Cable-Suspended Systems,” Oceanworks International, Inc, 1646 West Sam Houston Parkway N., Houston, Tex. 77043 (http://www.oceanworks.cc/whatsnew/paper.htmI)) describes three methods of compensation, which all lengthen or shorten the tow umbilical in unison with the vertical motion of the tow point on the surface vessel (for marine CSEM operations the tow point is the sheave 142 typically mounted on an ‘A’ frame 143 (FIG. 13) or other means of supporting the sheave such as a boom):
(i) Active compensation: Alternately paying out and taking up on the lifting winch 141 directly.
(ii) Active compensation: Moving the over-boarding sheave 142 at the end of the boom (or A-frame) up and down.
(iii) Passive compensation: Alternately stoking in and out idler sheaves (not shown) over which the lifting wire repeatedly passes.
The active techniques can achieve precise compensation but require more topside power, are technically and mechanically complex, are costly to install and maintain, and increase the number of repetitive cycles on the primary lifting device, which can reduce the mean time to failure.
II. Flying Wing Systems
Koterayama, et al. (“Motions of a Depth Controllable Towed Vehicle,” The Seventh International Conference on Offshore Mechanics and Arctic Engineering, Houston, Tex. (1988)) model a deep tow vehicle carrying CTD (Conductivity, Temperature and Depth) and ADCP (Acoustic Doppler Current Profiler) packages, which require stable depth and pitch/roll respectively. The deep tow vehicle was designed with wings and horizontal tails complete with feedback control. Static calculations and dynamic scale experiments demonstrate the wings and tail can control the operational depth and roll stability. The deep tow vehicle's pitch stability can be managed through the appropriate selection of the tow point. The authors conclude, “it has been demonstrated that the towed vehicle can not keep constant submerged depth without control of the wing when it is towed by the ship oscillating in waves [i.e., heave]. The attitude and submerged depth of the towed vehicle are very stable when it is under control [of the wing].”
III. Separate Depressor Systems
Wu and Chwang (“3-D Simulation of a Two-Part Underwater Towed System,” 1997 7th International Offshore and Polar Engineering Conference, Honolulu) discuss several numerical methods to dynamically model a two part deep tow system, which includes a depressor weight 144 attached to the umbilical 11 and the deep tow vehicle 145 towed by a secondary cable 146 attached to the primary umbilical (FIG. 13). Simulations illustrate the heave of the towed vehicle is attenuated as:
(i) The surface wave period decreases, and
(ii) The length of the secondary cable increases.
The authors conclude, “the results of numerical simulations indicate that the two-part tow method improves the behavior of the towed vehicle in vertical plane motions (heave and pitch), but no great difference is observed between the horizontal movements (surge and sway) of the towed vehicle and those of the depressor.”Buoyancy Control
EM survey contractors have attempted to control the pitch of the marine CSEM source dipole towed in a steady state by constructing neutrally buoyant streamers. The choice of hardware and their limitations are discussed next.
Flotation devices have been used in offshore and sub-sea activities for a number of years. Typical applications include:                (i) Sub-sea operations (including moorings and pipelines);        (ii) Seismic data acquisition (including cable floats);        (iii) Oceanographic data acquisition (including ADCP (Acoustic Doppler Current Profiler) floats); and        (iv) ROV/AUV applications (including buoyancy) [ROV: Remotely Operated Vehicles, AUV: Autonomous Undersea Vehicles].        
There are several different types of foam available for use in these flotation devices, including polyurethane, co-polymer and syntactic. Syntactic foam has the largest operational depth range and is normally selected for deep water applications (3,000 to 6,000 meters). Syntactic foams are low density composite materials which consist of miniature hollow glass spheres (10 to 300 microns in diameter) encased in a binding resin or polymer. The base polymer is chosen for a specific application (including operational depth and expected life). The glass spheres are added to reduce the specific gravity of the foam and increase the buoyancy.
Foam-based flotation devices are designed to provide minimum buoyancy for a given application over a specified depth range. Typical applications for foam floatation include:                (i) Provide buoyancy for ROVs,        (ii) Keep a cable or buoy at the sea surface,        (iii) Keep a tethered acoustic transponder off the seafloor, and        (iv) Provide sufficient buoyancy to return a device to the sea surface after release from a seafloor “clump weight” or anchor.        
Foam will compress with increasing pressure, which will reduce the available buoyancy. Syntactic foams are designed with very small compression ratios (typically ≦1.0% at operational depth). This reduction in buoyancy does not affect these normal applications. However, this small second order effect of buoyancy variation with water depth results in syntactic foam being a useful flotation medium for a marine CSEM source streamer, but not a complete solution. The flotation requirements for a marine CSEM source must cover all operational water depths (100 to 3,500+ meters). Commercial CSEM streamers are manufactured with a center core of strength members and electrical conductors surrounded by a foam collar. The design will yield a streamer that is nominally neutrally buoyant, but will not be neutrally buoyant over the entire operational water depth range.
Kerosene (a buoyant non-electrically conductive fluid) has also been used as a buoyancy medium in marine seismic streamers. The advantages of kerosene are:                (i) Availability;        (ii) Low cost (compared to foam); and        (iii) Essentially incompressible (retains buoyancy over CSEM operational range).        
U.S. Pat. No. 6,879,546 to Halvorsen, et al. lists disadvantages associated with kerosene-filled streamers:                (i) “Fluid-filled streamer cables suffer from a number of significant problems. The outer jacket is typically only a few millimeters thick and thus, is, easily penetrated by shark bites or other physical hazards encountered during towing, storage and deployment. Moreover, fluid-filled streamer cables are normally spooled onto large drums for storage on the vessels and often rupture during winding (spooling) and unwinding operations.”        (ii) “Seismic survey companies spend large amounts of money in repairing such cables and are typically forced to keep excessive inventory of such cables as spares for damaged cables. Outer jacket ruptures during surveying operations can require shut down of the surveying operations. Such down time can be very expensive due to the large capital cost of the vessels and the lost time of the crew, which can be several thousand dollars per hour.”        (iii) “Additionally, kerosene typically used in fluid filled streamers is toxic and highly flammable, which creates safety, health and environmental (SHE) problems. Moreover, streamer filler fluid leaking into the ocean is hazardous to marine life.”        
The outer jacket of a marine seismic streamer is thin to enhance the coupling of the embedded hydrophone with the surrounding sea water. Marine CSEM source streamers consisting solely of strength members and electrical conductors do not have the coupling issues associated with marine seismic streamers. Therefore outer jackets can be sufficiently thick and/or fabricated from a robust material to minimize jacket ruptures, but the resultant streamer must remain sufficiently flexible to be stored on a deck reel. While the small diameter and short length of CSEM streamers, relative to a typical (non-solid core) seismic streamer, reduces the volume of Kerosene stored on the vessel and deployed in the ocean, the toxic and fire hazards still exist.
Fielding and Lu disclose methods, including the use of thrusters and increased drag, for maintaining a vertical electric dipole in a vertical posture while being towed below the water surface, in PCT Patent Publication WO/2005/081719.